If a land affected by Winter's Night is untapped at the time its controller's next untap step begins, the "doesn't untap" effect doesn't do anything. It won't apply at some later time when that land is tapped.

10/1/2008

Winter's Night doesn't track the lands' controllers. If an affected land changes controllers before its old controller's next untap step, Winter's Night will prevent it from being untapped during its new controller's next untap step.

10/1/2008

This has the supertype world. When a world permanent enters the battlefield, any world permanents that were already on the battlefield are put into their owners' graveyards. This is a state-based action called the "world rule." The new world permanent stays on the battlefield. If two world permanents enter the battlefield at the same time, they're both put into their owners' graveyards.